1. Field of Invention
The present invention relates to a composition for use in pressed powder metallurgy.
2. Description of Related Art
In pressed powder metallurgy, substantially dry metal powders are placed into a rigid die cavity and pressed to form a green compact, which is then removed from the die and sintered at a temperature below the melting point of the major metallic constituent of the metal powder. Pressing causes the metal powder particles to mechanically interlock and form cold-weld bonds. Sintering strengthens the bond between the metal powder particles via solid-state diffusion.
Metal powders for use in pressed powder metallurgy are usually produced from high purity elemental metals and alloys. The metal powders are typically blended with lubricants and other additives, which serve to improve the handling characteristics of the unpressed metal powders and also facilitate the release of the pressed green compact from the walls of the die cavity.
Metal powders that have a high concentration of fines, which are generally defined as metal particles that are small enough to pass through a 325-mesh sieve, advantageously provide for relatively high density in the sintered metal part. However, use of metal powders having a high concentration of fines can be problematic. The fines tend to fall between the pin and die and galling tools, which can cause problems during processing. Moreover, such powders tend not to flow into the die cavity, as desired.
Some metal powders are very difficult, if not impossible, to use in conventional pressed powder metallurgy. For example, some inert gas atomized metal particles, which are substantially spherical in nature, provide insufficient green strength when pressed to allow for the removal of a green compact from the die. Moreover, metal powders consisting of a homogeneous blend of two or more metals or alloys having different specific gravities or particle sizes are difficult to press in conventional pressed powder metallurgy because the different powders tend to segregate rather than remain homogeneously mixed.